Reducing intersystem crossing rate of boron emitters for high-efficiency and long-lifetime deep-blue OLEDs

Blue organic light-emitting diodes (OLEDs) using boron emitters with narrow emission bandwidths hold great promise for next-generation high-resolution display applications. However, the challenge of designing deep-blue boron emitters that can achieve both excellent efficiency and superior operational stability persists. Herein, by integrating a tetrahydroquinoline type donor, a boron core and another pairing donor, a series of deep-blue emitters with asymmetric structures (B-N-S-1/2/3) were designed and synthesized. Their photophysical properties reveal that the donor units have a significant influence on the luminescent properties, including the emission wavelength, full-width at half-maximum and, especially, excited state kinetic constants. Notably, B-N-S-1 and B-N-S-2 exhibit typical thermally activated delayed fluorescence (TADF) characteristics and large intersystem crossing rate (kISC) constants, while B-N-S-3 demonstrates only prompt fluorescence emission with a small kISC of 0.38 × 108 s−1. In deep-blue OLEDs with an anthracene-based host, singlet (S1) excitons of the emitter could populate triplet (T1) excitons by intersystem crossing (ISC), which is then quenched by the host material bearing lower T1 energy, resulting in energy loss. In addition, the populated T1 excitons possess high emission energy, accelerating material decomposition. Hence, the deep-blue OLED based on B-N-S-3 with a reduced kISC shows a maximum external quantum efficiency of 6.7%, Commission Internationale de L’Eclairage color coordinates of (0.128, 0.119), and a notably extended operational lifetime (LT95) of 136 h, indicating that reducing the ISC rate of the emitters is a viable approach for enhancing the operational stability of blue fluorescent OLEDs.